1. Field of Invention
This invention relates to a vibrating type transducer and manufacturing process thereof; and more particularly, to a vibrating type transducer which is capable of generating stable self-oscillation and has high S/N ratio, and to a process for manufacturing such transducer.
2. Description of Prior Art
FIGS. 1, 2, 3 and 4 are block diagrams depicting one example of the prior art vibrating type transducer, wherein FIG. 1 is a perspective view of the transducer which is used as a pressure sensor, FIG. 2 is a block diagram wherein section A of FIG. 1 is enlarged and a vibration detection circuit is connected thereto, FIG. 3 is a sectional view taken along line A--A of FIG. 2, and FIG. 4 is an explanatory drawing showing an electrical equivalent circuit of the device of FIG. 2.
FIG. 1 depicts a silicon single crystal substrate 10 having a (100) plane on the top thereof, which is 10.sup.15 atoms/ cm.sup.3 or below, for example, in impurity concentration and of p-type conduction. A diaphragm 11 is formed from the back, through etching, as a thin layer on one side of substrate 10, as depicted.
A peripheral thick wall part 10 of diaphragm 11 is joined to a pedestal 14 having a pressure hole 13 at the center. Pedestal 14 has a pressure pipe 15 joined thereto so as to communicate with pressure hole 13. A pressure P (see arrow) to be measured, is introduced to pressure pipe 15.
An n+ diffusion layer (not indicated) having 10.sup.17 impurity concentration is formed partly on a surface of the side of diaphragm 11 indicated by reference letter A which is not etched. As shown in FIG. 2, a vibrator 16 is formed on a part of the n+ diffusion layer in the direction of &lt;001&gt;. Vibrator 16 is obtained, for example, by processing the n+ layer and the p-layer formed on diaphragm 11 using photolithography and underetching.
A magnet 17 is provided over vibrator 16 almost at the center thereof orthogonally to vibrator 16 and also positioned to not be in contact therewith. As shown in FIG. 3 an SiO.sub.2 film 18, used as an insulating film, is provided on layer 11, as depicted.
Metallic electrodes 19a, 19b (see FIG. 2), such as, for example, Al and the like, are depicted with one end of electrode 19a being connected to the n+ layer extending from vibrator 16, through a contact hole 20a provided by way of SiO.sub.2 layer, and with the other end of electrode 19a being connected through a lead wire (unnumbered) to a comparison resistance R.sub.o, almost equal to the resistance value of vibrator 16, and also to an input end of amplifier 21. An output signal is generated from an output end of amplifier 21, which is connected to one end of primary coil L.sub.1 of transformer 22. Another end of coil L.sub.1 is connected to common.
The other end of comparison resistance R.sub.o is connected to one end of a secondary coil L.sub.2 of transformer 22 with the midpoint thereof being connected to common, and the other end of secondary coil L.sub.2 being connected to the n+ layer through metallic electrode 19b and a contact hole 20b formed likewise on another end of vibrator 16
In the above device, when a reverse bias voltage is applied to the insulation between the p-type layer (i.e. substrate 10) and the n+ layer (i e. vibrator 16), and an alternating current is carried to vibrator 16, an impedance of vibrator 16 rises in a resonance state of vibrator 16. If the impedance is R, the equivalent circuit of FIG. 4, is obtained.
Secondary coil L.sub.2 having a center point C.sub.o connected to common, comparison resistance R.sub.o, and impedance R.sub.o together constitute a bridge. Thus, if an unbalanced signal, due to the bridge, is detected on amplifier 21 and the output is fed back positively to primary coil L.sub.1 through feedback line 23, the system will generate a self oscillation at a natural vibration frequency of vibrator 16.
The impedance R of vibrator 16 rises at the natural vibration frequency and may be expressed by the following: EQU R.apprxeq.(1/222).multidot.(1/Eg.gamma.).sup.1/2).multidot.(AB.sup.2 l.sup.2 /bh.sup.2).multidot.Q+Rd (1)
wherein E is the modulus of elasticity, g is gravity acceleration, .gamma. is the density of the vibrator material, A is the constant determined by vibration mode, B is the magnetic flux density, l is the length of vibration beam, b is the width of vibration beam, h is the thickness of the vibrator beam, Q is the quality factor, and Rd is the DC resistance value.
According to equation (1), since Q of vibrator 16 takes values of several hundreds to several tens of thousands, a large amplitude signal is obtainable as an output of amplifier 21 in the resonance state. Thus, by making the gain of amplifier 21 sufficiently large and by providing positive feedback, the system of the vibrating type transducer is self excited to vibration at the natural vibration frequency.
A p-type device obtained from diffusing, for example B (boron), on an n-type silicon substrate at 4.times.10.sup.19 atoms/cm.sup.3 or more, through selective etching may be used as a vibrator.
However, in such vibrating type transducer, a counter electromotive force generated on vibrator 16 is detected from an unbalanced voltage of the A.sub.C bridge. Since the component of an excited current cannot thoroughly be suppressed by the D.sub.C bridge, a voltage according to the excited current component is multiplied by the bridge output. Thus, the S/N ratio of the output is deteriorated by change in impedance of the vibrator being superposed on voltage of the excited component. Hence, stable output signal is not obtainable.